The Saint James Tornado: A Case Study of Boundary Interaction

On the evening of 1 June 1999, a tornado moved through portions of Saint James, Missouri. It produced F0 to F3 damage which destroyed 33 homes, and severely damaged the Missouri Department of Transportation maintenance facility. Use of WSR-88D algorithms was limited due to the range (approximately 93 NM) from which the tornadic storm developed. Although numerous moderate to strong mesocyclones were generated on the WSR-88D PUP before, during, and after the event, no TVS's were triggered beyond 80 NM on this storm. The pre-convective environment indicated a large CAPE (2700 J/kg), but relatively weak to moderate shear. Helicity values (185 m2/s2) were also marginal. Other meteorological factors which contributed to the St. James event were an increase in speed and depth of the environmental winds, topographic effects, and storm evolution through time. Numerous thunderstorms formed during the afternoon due to the strong instability. Outflow boundaries were also very apparent throughout the afternoon, but due to the proximity of the individual cells, storm inflow was relatively weak.

The apparent tornadic cell had a continuous moderate to strong mid-level mesocyclone one hour prior the event. Fifteen minutes before tornadogenesis, a collapsing supercell produced an outflow boundary that eventually intersected the apparent tornadic cell near Saint James, Missouri.

Since boundary interaction is becoming increasingly more crucial in severe weather nowcasting, radar operators should keep abreast of ever changing aspects of environment, and storm interaction during an event.

This case study will demonstrate the importance of assessing and monitoring mesoscale outflow boundary interaction and storm inflow (even at mid-levels) in pre-storm environment and during storm development and evolution.